check and prompt download if g++/clang++ is not available locally

[lennin331]

Problem

• When running cppx, the tool should check whether a C++ compiler (g++ or clang++) is available locally before attempting to compile the program.

• Currently, if neither compiler is installed or available in the system PATH, the user may encounter a low-level "command not found" or compilation error. This is especially inconvenient for the intended competitive programming workflow, where the user should be able to simply run: cppx followed by the file name (+flags) .

Expected Fix

• Look for the OS and suggest suitable download methods for clang++ or g++ (assuming executed on the native terminal)
• cppx install search and download latest cpp compilers.